The present invention relates to a conical gear having an involute tooth profile and a gear pair in which the conical gears are engaged.
In general, a bevel gear is used for engaging a plurality of gears which respectively rotate around a plurality of intersecting axes. In order to smoothly transmit rotation by the bevel gear, there are frequently used spiral bevel gears 103 and 104 having circular arc shaped teeth 101 and 102 and formed in a pseudo-spiral shape, as shown in FIG. 10(A).
In the manufacture of the bevel gears 103 and 104 mentioned above, the teeth 101 and 102 are machined one by one by a milling tool 107 in which a plurality of cutting chips 106 are arranged in an outer circumferential portion of a disc 105, as shown in FIG. 10(B). Accordingly, the working time is elongated and the working efficiency is lowered. Further, since a gear generating process is not employed, it is hard to form the involute tooth profile. Accordingly, the involute tooth profile may be formed by using a formed cutting blade or grinding stone in place of the milling tool 107. However, in the teeth 101 and 102 of both of the bevel gears 103 and 104, since the diameter of an inside circular arc is different from the diameter of an outside circular arc, a precise engaging position between the bevel gears 103 and 104 exists only at one position in an axial direction of the bevel gears 103 and 104 even if the involute tooth profile is obtained. Accordingly, in order to precisely engage the bevel gears 103 and 104, a high assembling precision is required.
In addition, as shown in FIG. 10(C), since one tooth surface of both tooth surfaces of each of the teeth 101 and 102 is convex, and the other tooth surface is concave, a concavo-convex relationship between a drive tooth surface and a driven tooth surface is counterchanged by reversing rotating directions of both bevel gears 103 and 104. Accordingly, if the rotating direction of the gear is different, a great difference is generated in a transmission efficiency. In the case of the gear having no involute tooth profile, the transmission efficiency is further lowered.
Further, as is apparent from FIG. 10(A), in the case where a crossing angle (δ1 +δ2) between the axes of the bevel gears 103 and 104 is small, a distance (hereinafter, referred to as a con-distance) R between large diameter ends of both the bevel gears 103 and 104 and a conical center thereof is elongated. Each of the bevel gears 103 and 104 is supported on a rotating table at a time of being machined. Further, each of the teeth 101 and 102 is cut by the milling tool 107 while rotating the table in increments of one pitch of each of the teeth 101 and 102. Accordingly, the longer the con-distance R becomes, the larger the radius of rotation of the table must be. As a result, a wide working space is necessary.
On the other hand, Japanese Laid-Open Patent Publication No. 6-94101 discloses a conical involute gear which can be machined by a gear hobbing machine. The conical involute gear disclosed in this publication has an involute tooth profile machined by a gear generating process. Further, as shown in FIG. 11, one gear 311 in two gears constituting a gear pair is conical, and the other gear 312 is cylindrical. Further, as described in the specification of this publication, in order to engage the gears 311 and 312 in a wide range, an addendum modification coefficient of the conical gear 311 is changed in such a manner as to draw a non-linear gentle curve in a face width direction of the gear 311. Accordingly, it is possible to bring the tooth surfaces of both the gears 311 and 312 into contact with each other approximately over a whole face width.
However, in each of the gears 311 and 321 disclosed in Japanese Laid-Open Patent Publication No. 6-94101, as is apparent from a numerical expression (8) and a numerical expression (9) described in the specification of the publication, the addendum modification coefficient is set by using numbers of teeth z1 and z2 of the respective gears 311 and 321 as they are. The use of the numbers of teeth z1 and z2 as they are for setting the addendum modification coefficient does not express the teeth perpendicular to the axis of the conical gear as is apparent from FIG. 1B of the present application and FIG. 4 of Japanese Laid-Open Patent Publication No. 6-94101, but the teeth perpendicular to the conical surface of the conical gear. Since each of the gears 311 and 321 is rotated around its own axis, a relationship between the center of rotation of each of the gears and the engagement of the tooth of each of the gears is not established even if the teeth perpendicular to the conical surface of the respective gears 311 and 321 are engaged. Accordingly, it is impossible to smoothly engage the gears, and it is impossible to achieve a suitable engaging function as the involute gear. In addition, Japanese Laid-Open Patent Publication No. 6-94101 does not mention an allocation of the addendum modification coefficient of both the gears 311 and 321. Therefore, the engagement between both the gears 311 and 321 is partly established and comes close to a point contact, and it is impossible to achieve a smooth rotation transmission.
Further, Japanese Laid-Open Patent Publication No. 6-94101 only refers to a positive addendum modification from a reference pitch point P0 positioned in a small diameter end of the gear, in connection with a change of the addendum modification coefficient, and does not take into consideration positive addendum modification and negative addendum modification. Therefore, it is not possible to sufficiently correspond to a great change of the addendum modification coefficient and it is hard to achieve the conical gear having a large conical angle.